This invention relates generally to the introduction of eluted sample components into a gas chromatography detector and more particularly to an apparatus and technique for switching between the introduction of pure carrier gas and sample components in a kathorometer.
Generally, in gas chromatography, the sample to be analyzed is introduced into the chromatography column in a stream of carrier gas. The separation process takes place in the column and at the end of the column the individual components will emerge more or less separated in time. The individual components separated by the column are detected by continuously monitoring some physical or chemical properties of the effluent.
Ideally, each component in the sample emerges from the column at different times so that, at any one time, the gas flowing into the detector is either all carrier gas or a combination of carrier gas and one of the components of the sample. The detector functions by producing a signal related to the change in the intensity of a given characteristic of the gases flowing through it. As each sample component passes through the detector, the output signal varies from the value it has when the detector is full of carrier gas, with the amount of variation depending on the concentration of the sample component.
A widely used detector is the thermal conductivity detector (also referred to as a hot wire detector or katharometer) which measures the differences in the thermal conductivity of the pure carrier gas and the mixture of the sample component and the carrier gas. Other types of detectors include the flame ionization detector, the electron capture detector, the thermionic detector and the flame photometric detector.
A typical thermal conductivity detector includes a block having a cavity therein with a filament suspended in the cavity and ports at either end of the filament. One of the ports is connected to the outlet end of the chromatogrphy column. Current is passed through the filament so as to heat it and means are generally provided for maintaining the block and therefore the walls of the cavity at a fixed temperature that is less than the temperature of the filament. The output signal of the detector corresponds to the variation in voltage applied to the filament or the current flowing through it that are required to keep the filament at a given temperature or resistance. The temperature of the filament depends on the rate at which heat can flow from it to the walls of the cavity. Nearly all the heat flows by conduction through the gases between the filament and the walls of the cavity. Generally, helium is used as a carrier gas because of its inertness and the fact that its thermal conductivity is greater than that of all gasses except hydrogen.
In operation, the flow of gas through the detector is switched between pure carrier gas (for reference measurements) and effluent from the column. Such switching may be accomplished by mechanical switching arrangements such as the rotary valve configuration disclosed in Kolloff, U.S. Pat. No. 4,464,925 for HYDROGEN, DETERIUM THERMALCONDUCTIVITY DETECTOR issued Aug. 4, 1984 and incorporated herein by reference. This configuration utilizes a motor and rotary valve to accomplish the desired switching.
It is an object of the present invention to provide a new and improved gas chromatography system.
Another object of the invention is to provide a new and improved apparatus and technique for alternately filling the detector of such a system with carrier gas and column effluent.
A further object of the invention is to provide such an apparatus for rapidly switching between carrier gas and effluent which does not utilize moving parts.
Another object is to provide such an apparatus which is economical, reliable and efficient.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
Accordingly, it has been found that the foregoing and related objects and advantages can be obtained in a gas chromatography system having a chromatography column and a detector with a detector chamber connected to the column outlet. A switching assembly for selectively, alternately filling the detector chamber with carrier gas for a baseline measurement and with effluent for analysis is connected to the detector and includes a gas chamber connected to the detector chamber, a source of carrier gas connected to the gas chamber, and apparatus for selectively heating gas in the gas chamber to cause carrier gas to be expelled from the gas chamber to fill the detection chamber for a baseline measurement and for cooling gas in the gas chamber to withdraw the carrier gas from the detection chamber so as to draw effluent from the column outlet to thereby fill the detection chamber for an analytical measurement. The heating apparatus includes a hot wire filament mounted in the gas chamber and electrically controlled for selective heating.